1. Field of the Invention
This invention relates to plug lids for containers and more particular but not exclusively to a plug lid made of plastics material and adapted to fit in a plastics ring component of a container body.
2. Description of Related Art
Paint cans have traditionally been made out of tinplate to comprise a cylindrical body member to which are seamed a base closure member and an upper ring designed to receive a plug-fit tinplate lid. The performance of this lid has relied on there being an interference fit between the plug wall and the ring, the assembly operation therefore demanding elastic deformation of these components to achieve adequate lid retention during storage and transit.
Because of a variety of problems associated with the use of tinplate for such containers, including their ease of denting and the difficulty in preventing their internal rusting when containing water-based products, which now form the majority of all paint products, manufacturers have turned increasingly to the use of plastic containers which eliminate these costly disadvantages.
Generally such containers are made of either polypropylene or polyethylene to satisfactorily contain the water-based paints and are often injection moulded in one operation to incorporate base, body and ring. Alternatively, a cylindrical-walled pot may be moulded separately from the ring which is subsequently joined to the pot using an appropriate welding operation.
This fabricated pot and ring is used where there is a market requirement for a cylindrical-walled container that resembles the metal can in appearance and a concommittant implication is that the lid will be of a plug fit type rather than of an over-fitting type that generally extend radially beyond the wall of the container.
Now, because of the elastic nature of the plastic containers and their generally low coefficient of static and dynamic friction, the plug fit lids referred to above cannot rely solely on interference fit to resist the outward thrust of contents but require a radially-projecting bead on their wall portion, designed to hook beneath a correspondingly designed arrest feature on the ring.
An arrangement of ring and lid fit is described in British patent No. 2150540 wherein the retention bead of the lid projects radially outwards underneath the sealing lip of the ring. Of necessity, either the ring or lid or both has to accept and accommodate elastic deformation in order to achieve the assembled configuration shown and it is precisely because of this ability to deform and the inadequacy of design that plastic lids to date have shown very poor retention characteristics when subjected to the thrust of contained fluid contents arising when a container is dropped onto a surface.